1. Field of the Invention
The present invention relates generally to a light modulator type multi-beam scanning apparatus and, more particularly, to a light modulator type multi-beam scanning apparatus using a dichroic slit, in which each of the transmissive portions of the dichroic slit, which corresponds to light of a specific color, is coated with a material capable of transmitting only light of the specific color so as to filter light of various colors using a single slit.
2. Description of the Related Art
A light beam scanning apparatus is an apparatus that focuses an image by scanning a light beam and spotting the light beam on a photosensitive medium in an image producing apparatus, such as a laser printer, a display device, a Light Emitting Diode (LED) printer, an electronic photocopier or a word processor.
As image producing apparatuses have trended toward small size, high speed and high resolution, light beam scanning apparatuses have continuously been developed to have small size, high speed and high resolution characteristics correspondingly.
Light beam scanning apparatuses for image producing apparatuses may be classified into a laser scanning type using an f·θ lens and an image head printer type according to a light beam scanning method and the construction of a light beam scanning apparatus.
FIG. 1 is a perspective view showing a conventional scanning apparatus using a single light sources and an f·θ lens.
As shown in FIG. 1, the conventional laser scanning apparatus includes a Laser Diode (LD) 10 for emitting a light beam in response to a video signal, a collimator lens 11 for converting the light beam, emitted from the LD 10, into collimated light, a cylinder lens 12 for converting the collimated light, having passed through the collimator lens 11, into linear light parallel to a scanning direction, a polygon mirror 13 for scanning the linear light, having passed through the cylinder lens 12, by moving the linear light at a constant linear velocity, a polygon mirror drive motor 14 for rotating the polygon mirror 13 at a constant velocity, an f·θ lens 15 having a constant refractive index with respect to a light axis and focusing constant angular velocity light by deflecting the constant angular velocity light, reflected by the polygon mirror 13, toward a main scanning direction and correcting aberration, a reflecting mirror 16 for forming an image on the surface of a photosensitive drum 17 by reflecting the light beam, transmitted through the f·θ lens 15, toward a predetermined direction, a horizontal synchronization mirror 18 for reflecting the light beam, transmitted through the f·θ lens 15, toward a horizontal direction, and a light sensor 19 for receiving the light beam, reflected by the horizontal synchronization mirror 18, and performing synchronization on the light beam.
The laser scanning type light beam scanning apparatus is problematic in that it is difficult to achieve high-speed printing due to the low switching speed of the LD 10 and the scanning speed of the polygon mirror 13.
Meanwhile, to increase the speed at which the light beam is scanned, the polygon mirror 13 must be rotated using a high-speed motor. In this case, the high-speed motor is costly, and deteriorates the operational reliability of the laser scanning apparatus by generating heat, vibration and noise, so that it is difficult to increase the scanning speed by employing the high-speed motor.
Another method of increasing the speed of the light beam scanning apparatus is an image head printing method using a multi-beam producing apparatus.
A multi-beam scanning apparatus has a plurality of light emitting units (laser heads). The multi-beam scanning apparatus simultaneously and optically scans a plurality of light beams, emitted from the plurality of light emitting units, onto the surface of a storage medium in the form of a plurality of light spots in such a way as to image the plurality of light beams, transmitted through a light reflector, using an imaging lens.
Although the number of times per unit time that a single light spot is optically scanned onto the surface of the storage medium must be considerably increased to achieve high-speed printing using the single light spot, the rotational speed of the light reflector and an image clock cannot meet that high number of times per unit time. Consequently, when the number of beam spots increases, the rotational speed of the light reflector and the image clock is in reverse proportion to the increase in the number of beam spots.
To effectively form a plurality of beam spots, a laser element functioning as a light source can have a plurality of light emitting points (light emitting units) that can be independently operated.
Such a laser element having a plurality of light emitting points is generally referred to as a “monolithic multi-beam laser element.” When the monolithic multi-beam laser element is used, various optical elements arranged behind the light source can be generally used in conjunction with a plurality of light beams, so that the monolithic multi-beam laser element provides great advantages in cost, work efficiency and adjustment.
FIG. 2 is a diagram showing a conventional image head printing method for performing laser scanning using a multi-beam formed by an LED array constructed in an image head.
Referring to FIG. 2, by constructing an LED array 21, in which the number of LEDs 22 is so large that the LEDs 22 can fill a printing paper, in an image head 20, the conventional image head printing method can print one line at one time without the use of a polygon mirror and an f·θ lens, unlike the laser scanning method, thus increasing printing speed considerably.
Such a monolithic multi-beam laser element includes, for example, a so-called surface emitting laser (surface emitting semiconductor laser).
A surface emitting laser emits a light beam in the direction parallel to the transverse direction of a silicon layer, whereas a conventional semiconductor laser emits a light beam in the direction vertical to the transverse direction of the silicon layer.
The surface emitting laser has the following characteristics.
The conventional semiconductor laser emits light having an elliptical cross section and various divergence angles, whereas the surface emitting laser can emit a circular beam having a stable divergence angle.
However, the surface emitting laser is problematic in that its output light beam has an unstable polarization direction. Even though the polarization direction can be somewhat controlled by a manufacturing process, it varies with a light emitting point, a surrounding temperature, and output.
In general, the reflectance, transmittance and angle characteristics of the optical elements of an optical scanning apparatus, such as a polygon mirror including a light reflector, a scanning lens functioning as an imaging optical system, and an echo mirror changing an optical path, vary with the polarization direction of an input light beam.
For this reason, when a monolithic multi-beam laser element including the surface emitting laser is used as the light source of an optical scanning apparatus, a plurality of beam spots scanned on the surface of a storage medium have different intensities according to the different polarization directions of light emitting points of the beam spots. The difference in the intensity causes non-uniformity in pitch on an image, thus reducing image quality considerably.